Search: optimize excerpt

[lookupdaily]

Original search PR: #984
Related issues: #1025 (match-counter) #1030 (optimize search)

An excerpt for a search result is often not most relevant excerpt, which may cause a user to dismiss a result as irrelevant. Currently the excerpt is generated by finding the match index for the first matching word in the search term, but does not prioritise full matches (likely to be a more relevant result).

When you search for multiple terms it should try to find a match for the whole search phrase first, then fall back on matches for constituent terms if there's no whole-phrase match. This should make it a little clearer when results are actually relevant, but just part of very long pages.

This is linked somewhat to #1025, which suggests a match-counter for each result so that the user will be able to easily see that there are other relevant results on the same page if the except text is not what the user is looking for.